Control systems

ABSTRACT

A multi-point control system suitable for applications where a large number of processes, typically in excess of 100, are all required to be stabilized at the same operating conditions. The control system comprises on-off controllers fed with a common set-point signal. The controllers being adapted to function as time-proportional controllers by the perturbation of either the set-point signal or the process parameter signal by a periodic waveform. Other signals may be combined with the perturbing signal to provide, for example, compensation for a fluctuating power supply, or for environmental conditions affecting the deviations at the inputs of controllers. Such a system has economical advantages and by slight modifications to the controllers, the system can be used for differing applications.

United States Patent 1451 Oct. 10,1972

Johnston CONTROL SYSTEMS [72] inventor: James Stewart Johnston, Sussex,En-

- gland v [73] Assignee: Rosemount Engineering Company,

- Limited, Bognor Regis, Sussex, En-

gland 1 [22] Filed: Feb. 10, 1971 [21] Appl. No.: 114,294

US. Cl. ..307/3l, 235/15 1.21, 307/l52 Primary Examiner-Herman J.Hohauser Attorney-Bugger, Peterson, Johnson & Westman [57] ABSTRACT Amulti-point control system suitable for applications where a largenumber of processes, typically in excess of I00, are all required to bestabilized at the same operating conditions. The control systemcomprises on-ofi controllers fed with a common set-point signal. Thecontrollers being adapted to function as time-proportional controllersby the perturbation of either the set-point signal or the processparameter signal by a periodic waveform. Other signals may be combinedwith the perturbing signal to provide, for example, compensation for afluctuating power supply, or for environmental conditions affecting thedeviations at the inputs of controllers. Such a system has economicaladvantages and by slight modifications to the controllers, the systemcan be used for differing applica- 20 Claims, 8 Drawing Figures 1 5111111.01 ..,n02 1/o0 [58] Field of Search ..307/38, 11, 31, 35, 149, 152;1 23s/151.1,1s1.21

[56] References Cited UNlTED STATES PATENTS 3,514,691 5/1970. Levinetal...307/31x 3,629,562 12/1971 Davisetal; ..307/31xr 4 DIFFERENTIALAMPLIFIER 31. ,c 11 12 1a d GATE rmooz'remo E UNIT PATENTED BUT I 0 I9728. 6 97. 768

SHEEI10F4 32 b DIFFERENTIAL m AMPLIFIER I ER.

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32 DIFFERENTIAL sc R 13 GATE TRIGGERING UNIT ALARM- C DIFFERENTIALAMPLIFIER 7 SET PUINT Fig 8 SUURL'E \IJ PERTURBATION vb] SIGNAL. souRcE[1 /THERMOCOLAPLE 1, COMPENSATION 'C coMPENsAToR BACKGROUND OF THEINVENTION This invention relates to control systems in which a number ofindividual electrical controllers are combined to provide set-point andtime proportional control-of one or more process variables in responseto measurement information.

An example of an application for such a control system is the control ofa large number of heaters, typically in excess of 100, of a nylon drawtwist machine in the textile industry, where it is required to stabilizeall the heaters at the same temperature.

It is known for such applications to use a large number of proportionalcontrollers but this results in great expense. It is thereforepreferable to use on-off.

controllers for means of economy, however it is still desirable to use aproportional control for the stabilization of the process parameters.

Thus for reasons of economy and versatility it is desirable to reducethe amount of circuitry of the individual controllers and to remove anycomplex circuitry of the individual controllers, e.g., set point source,and incorporate it in a central purpose built circuit. This enables easyinterchange of controllers with different control functions fordiffering applications.

SUMMARY OF THE INVENTION According to this-invention there is provided acontrol system for'individually controlling the application of a powersupply whereby each of a number of processes can be controlled inaccordance with the deviation of the value of a sensed variableparameter of each process from a common set point for the processes,which system comprises a multi-line bus, a controller for each processin the form of a unit connected to said bus, a common power supplysource for the controllers feeding common power supply lines in saidbus, a common set-point signal source feeding a set-point line in saidbus, a common perturbation signal source supplying a periodicallyvarying electrical signal to a perturbation line in said bus, eachcontroller comprising a high gain differential amplifier having a firstinput, a second input and an output, a sensing device providing aprocess variable parameter signal, means for connecting said variableparameter signal to said first input via a first resistance, means forconnecting said perturbation line to said first input via a secondresistance, said second input being connected to said setpoint line insaid bus, and switching means for controlling the application of saidpower supply for one of said processes, said switching means having acontrol input fedby the output of said differential amplifier.

The choice of power supply for control of a process will depend upon theprocess, for example, in one case it may be required to control a heaterto stabilize an elevated temperature therefore a convenient power sourcewould be an external mains supply, and in another case it may berequired to stabilize the temperature of a refrigerated process and itmay be convenient to control the flow of a coolant by actuating asolenoid valve which may be powered by an external or internal D.C.supply, which may even be the same supply as that used for thecontrollers. Therefore it can be seen that the power supply may beexternal or internal, A.C. or DC, or even the common power supply forthe controllers.

A typical controller may comprise a differential amplifier provided witha number of inputs of various impedances and an output connected toswitching means for controlling the supply of electrical power to aload. The switching means may include a'silicon controlled rectifier,such as a thyristor, which is fired by pulses from triggering meansturned on and off by the output of the differential amplifier; Thetriggering means may be a blocking oscillator, or a circuitincorporating a reed switch or relay. Alternatively the switching meansI may simply comprise a relay with its relay contacts arranged in serieswith the load, its relay coil connected between the output of thedifferential amplifier and a terminal of a DC supply, there beingprovided means for permitting the relay coil to be energized for onlyone sense of said deviation.

In the case where a thyristor is to switch an A.C. supply to a load, itmay be preferable to feed the output of the differential amplifier tothe triggering means via an AND gate which is controlled by a gatingsignal so that the thyristor can only be fired at or near the zerovoltage points of the voltage waveform of the A.C. supply to preventradio frequency interference.

Conveniently each controller may be .mounted on substantially identicalcircuit cards each provided with a series of input and output sitesarranged on one edge of the card.

When temperature is one of the measured variables, the appropriatecontroller circuit may be provided with input impedances connected in ahalf-bridge configuration one arm of which includes a resistancethermometer for measuring the temperature.

A set-point signal may comprise a DC voltage level derived from apotential divider circuit connected across a common DC. power supply andmounted on a separate circuit card.

Time-proportional control may be effected by generating on a furthercircuit card a periodically varying electrical signal, such as atriangular voltage waveform, and feeding this signal down one of thecommon supply lines to the inputs of appropriate controllers, wherebymeasurement signals to each controller are modulated and the switchingmeans is switched on and off at a frequency equal to the frequency ofthe time varying electrical signal. The duration of the on and offperiods is thus dependent upon the value of the measurement signal toeach controller.

According to this invention there is further provided a control systemforindividually controlling the application of a power supply wherebyeach of a number of processes can be controlled in accordance with .thedeviation of the value of a sensed variable parameter of each processfrom a common setpoint for the processes, which system comprises amultiline bus, a controller for each process in the form of a unitconnected to said bus, a common power supplysource for the controllersfeeding common power supply lines in said bus, a common set-point signalsource, a common perturbation signal source, summing means for combiningsaid set-point signal and said perturbation signal and for feeding acombined signal onto a set-point line in said bus, each controllercomprising a high gain differential amplifier having a first input, asecond input and an output, a sensing device providing a processvariable parameter signal, means for connecting said variable parametersignal to said first input, said second input being connected to saidset-point line in said bus, and switching means for controlling theapplication of said power supply to one of said processes, saidswitching means having a control input fed by the output of saiddifferential amplifier.

There may also be provided at least one means for providing a commoncorrection signal which is fed to said summing means. The correctionsignal may be derived from fluctuations of the power supply or from theaverage of the deviations of the inputs of the controllers.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 shows a schematic circuitdiagram of basic controller card;

FIG. 2 shows the basic controller card of FIG. 1 used in a system with acommon set-point to provide on/off control;

FIG. 3 shows the use of a generator with the arrangement of FIG. 2 toprovide time proportional control;

FIG. 4 showsa basic controller card in which a relay is used as aswitching element in place of a thyristor used in the basic controllercard shown in FIG. 1',

FIG. 5 shows a simplified controller card of the kind shown in FIG. 1 inwhich there is provision for only one input for example for. theset-point signal and any further input signals are introduced bymodulating this DESCRIPTION OF THE PREFERRED EMBODIMENT Referring toFIG. 1 a card 21 carries components for a simple controller circuit inwhich a high gain differential amplifier has a first input connected toterminal d and a second input connected to terminals a, b, 01, c2, c3and 04 via impedances 31, 32, 33. An AND gate unit 11 is connected tothe output of the differential amplifier and controls the operation of atriggering unit 12 which in turn controls the firing of a thyristorswitching unit 13. In one arrangement the triggering unit may be ablocking oscillator, in another it may comprise a reed switch or relay.A pair of terminals 18 are connected to the thyristor which can then beused to pass or block cycles of an external mains power supply connectedto series with a load to be controlled. The input connections toterminals cl to c4 are so arranged that a half-bridge circuit can easilybe formed, in one arm of which may be connected a resistance thermometer34. Inhibiting of the gate 11 may be effected via a control terminal e.Stabilized 12v power supplies are connected via terminals 16 and 17 andcommon terminal for the internal controller circuitry.

In FIG. 2 a basic controller card 21 similar to the kind shown in FIG. 1is provided with a set-point signal generated on another circuit card 25using a DC. voltage signal derived from ,a potential divider chainbetween the 12v and common supply lines. A series of supply lines 30 isused to interconnect the cards 21,

25, one line being used for one terminal of the basic card as requiredi.e. line a supplying all terminals a. This set-point voltage signalforms the first input to the differential amplifier 10 on terminal d.

Terminals c3 of all cards and the corresponding end of the potentiometerin the set-point signal source are connected to line 0'. This is tosatisfy the bridge conditions for each bridge formed by a sensorhalf-bridge network and the half-bridge network of the set-point source,the amplifier of each controller acting as the detector for its ownbridge network.

The differential amplifier 10 produces an output dependent upon thedeviation of the actual value of the measured variable, in this casetemperature, from the desired value of that variable, represented by thesetpoint voltage signal. For some applications one input to the AND gate11 may be permanently connected to a positive voltage so that thetriggering unit 12 will start firing as soon as the measured temperaturefalls below the set-point. The thyristor 13 will be maintained inconduction all the time that the temperature is below the set-point andwill be switched off whenever it is above. With this arrangement on/offcontrol of a single process variable, in this case temperature, may beachieved.

Using the circuit arrangement shown in FIG. 3 time proportional controlmaybe superimposed upon the set-point control achieved with the circuitarrangement shown in FIG. 2. This means that the duration of any controlaction resulting from comparisons between actual or measured values anddesired or set-point values of a process variable can be varied byturning the controller successively on and off as required. In this casethe variation is in synchronlsm with a timing source which comprises atriangular waveform generator 24 mounted on another circuit card 26. Thetriangular waveform has a period of about one second and is fed along acommon supply line a to perturb the existing measurement signal, forexample temperature derived from a resistance thermometer 34 at thesecond input to the differential amplifier 10 on controller card 21.

The peak-to-peak amplitude of the triangular waveform defines theallowable range of variation of the measurement signals for the timeproportional control to be possible. In some applications, e.g., controlof an elevated value of a parameter such as an above ambienttemperature, control action is initiated when the measured value isbelow the set-point, however in other applications it may be required tocontrol a depressed value of a parameter, e.g., the temperature in arefrigerator, and in this case control action is initiated when themeasured value is above the set-point. The correct output can beobtained from the amplifier for both these types of application byappropriate connection of the two inputs of the differential amplifier.With an elevated temperature as the measured variable the controllerwill be on" for temperatures below the setpoint and of for temperaturesabove the set-point. Thus the allowable range of variation of measuredtemperature for time proportional control has as an upper limit, theset-point, and as a lower limit, the set-point minus the peak-to-peakamplitude of the triangular waveform. For measured values below thelower limit the controller will not turn of under the time modulationaction of the triangular waveform and for measured values above theupper limit the controller will be continuously off. When the measuredvariable, temperature, lies within the allowable range the controlleroutput will comprise alternate periods of conduction and non-conductionof the thyristor 13 at a fixed frequency equal to the frequency of thetriangular waveform but with a variable mark/space ratio depending onthe particular value of the temperature. A disadvantage with thethyristor switching arrangement of FIG. 3 is that with such siliconcontrolled rectifiers which are in conduction for blocks of successivecycles, in the first cycle of conduction the point at which conductionstarts is uncontrolled, since it depends only on the instant at whichthe measured temperature drops below the set-point. This can result in aradio-frequency interference signal which can be prevented byintroducing a further waveform onto line e instead of the l2v D.C.connection shown in FIGS. 2 and 3, which waveform remains at zeropotential except for the first millisecond of each half-cycle of theexternal mains supply. This restricts the initiation of conduction ofthe thyristor to within this period and consequently if the sum of themeasured value and the perturbation signal falls below the set-point atan instant halfway through a mains half-cycle, conduction in thethyristor will not start until the beginning of the next half-cycle.Thereafter means half-cycles will be passed in bursts of the frequencyof the perturbation signal until the end of the mains half-cyclefollowing the instant the sum of these two signals lies wholly above theset-point.

If required, phase-angle control of the thyristor switching can beachieved by replacing the triangular waveform of 1 second period by oneof 10 milliseconds period (for 50 Hz mains frequency) synchronized withthe cycles of the external mains supply. In this case the gate would beheld fully open by a permanent connection to the positive line 17.

If the mains power supply for the loads is likely to be disturbed,resulting in an equal supply fluctuation for each controller card, asignal derived from measurement of this disturbance may be introduced or-fedforward along b to change the effective input setting and henceoutput on all controllers simultaneously. Alternatively, such a commonfeed-forward signal may be produced by the computation of the averagedeviation exhibited on all controller card channels. This could be doneby sequentially scanning the various controller inputs or by connectionof an averaging network to the individual measuring points integratingthis mean error and feeding it to all controllers along line b. Thiscommon integral action would not eliminate independent errors onindividual controllers but in many systems environmental power supplyand other factors affect all controllers equally.

Although it is generally desirable to keep individual controllers assimple as possible a certain amount of adjustment of operational rangeof input signals for each controller may be effected by arranging forresistor 31 to be variable whereby the proportion of the modulatingwaveform fed to that controller may be adjusted. It would even bepossible to have one of these controllers operating from itsownset-point while still receiving the modulating waveform common to theremaining controllers. However, any increase in the number ofindependent controls for each controller tends to reduce thecompatibility of the controllers with-a common control system.

The modulating waveform used for time proportional control need notnecessarily be the triangular waveform previously described, but may forexample, be shaped so as to provide deviation dependent sensitivity or alinear input/output characteristic in a phase-angle switching or S.C.R.firing system. I

Further the switching element need not be a thyristor or indeed any formof semiconductor controlled rectifier but, as shown in FIG. 4, may be asimple relay 28 with switch contacts 29, the relay being in series withdiode 35. Alternatively the output of the amplifier may be clamped ineither the positive or negative direction. The circuit of FIG. 4 isfully compatible with the common supply rails 30 and can be used foron/off or time proportional control or for example as shown in FIG. 6 asa monitoring system to switch on an alarm device if the measured valuedeviates too sharply from the setpoint.

When the measured variable is temperature a thermocouple 46 may be usedin place of a resistance thermometer as shown in FIG. 7, and for morethan one controller, the cold-junction compensation voltage would besupplied to all appropriate controllers by using it to adjust theset-point voltage.

In FIGS. 1, 2, 3 and 4 there are shown a number of feed-in resistors forthe second input of the differential amplifier 10 and signals to performa variety of functions may be fed along these various resistors.However, as shown in FIG. 5, it is possible to use only one inputconnected to terminal d and a single supply rail (not shown), normallyreserved for the set-point voltage.

FIG. 8 shows an arrangement in which a set-point signal from a set-pointsignal source 47 is combined with a number of other signals before beingfed to the set-point line d. The set-point signal is fed to one input ofa summing device 40. A perturbation signal source 41 is connected toanother input of the summing device 40. Other inputs may be fed, asrequired, from a thermocouple cold-junction compensation source 42, acompensator 43 which provides a compensation signal dependent uponfluctuations of the power supply for the loads, and a compensator 44which provides a compensation signal derived from the time average ofthe deviations of the inputs of the controllers.

The set-point signal source 47 may be identical with the set-pointsignal source 25 shown in FIGS. 2 and 3, but if desired may be of anyother suitable circuit for providing an adjustable set-point signal, ormay even be a circuit for processing or modifying a reference signalsupplied by associated equipment. In the case where the sensor networkis a half-bridge, these alternative circuits must produce a set-pointvoltage with respect to the voltage on the line 0 connected to allpoints c3 of the controllers. If the sensor is a thermocouple, thenthese alternative circuits must produce a set-point voltage with respectto the reference voltage for the thermocouple, which may be thezero-volt power supply line as shown in FIG. 7.

Some embodiments of this invention find particular application in thecontrol of textile machinery.

Iclaim:

l. A control system for individually controlling the application of apower supply whereby each of a number of processes can be controlled inaccordance with the deviation of the value of a sensed variableparameter of each process from a common set-point for the processes,which system comprises a multi-line bus, a controller for each processin the form of a unit connected to said bus, a common power supplysource for the controllers feeding common power supply lines in saidbus, a common set-point signal source feeding a set-point line in saidbus, a common perturbation signal source supplying a periodicallyvarying electrical signal to a perturbation line in said bus, eachcontroller comprising a high gain differential amplifier-having a firstinput, a second input and an output, a sensing device providing aprocess variable parameter signal, means for connecting said variableparameter signal to said first input via a first resistance, means forconnecting said perturbation line to said first input via a secondresistance, said second input being connected to said set point line insaid bus, and switching means for controlling the application of saidpower supply for one of said processes, said switching means having acontrol input fed by the output of said differential amplifier.

2. A control system as defined in claim 1 wherein there are providedmeans for sensing a disturbance of said power supply for said processesand for feeding a first common correction signal to a correction signalline in said bus, and means for connecting said correction signal lineto the first input of each controller via a third resistance.

3. A control system as defined in claim 1 wherein there are providedmeans for averaging said deviations, for deriving a second commoncorrection signal from said averaged deviations, and for feeding saidsecond common correction signal to another correction signal line insaid bus, and means for connecting said correction signal line to thefirst input of each controller via another third resistance.

4. A control system as defined in claim 1 wherein there is provided forat least one controllera variable resistance sensor having one of itstwo ends connected via a fourth resistance to one of said common powersupply lines, its other end connected via a fifth resistance to anotherof said common power supply lines, and a sixth resistance connectedbetween one end of the or each sensor resistance and said first input ofthe or each amplifier, so that the output of the or each amplifier is ofthe required sense for control of said parameter.

5. A control system as defined in claim 4 wherein said set-point signalsource comprises a potentiometer having one of its ends connected by aseventh resistor to said one of said common power supply lines and itsother end connected by an eighth resistance to said another of saidcommon power supply lines, the common set-point signal being fed to theset-point line in said bus from a slider of the potentiometer, andwherein said other end of the or each sensor is connected to said otherend of said potentiometer via a line in said bus.

6. A control system as defined in claim 1 wherein there .is provided forat least one controller a thermocouple having at least one hot and coldjunction, the thermocouple having an output voltage fed to the firstinput of the amplifier, and means for combining a cold junctioncompensation voltage with said set-point signal.

7. A control system as defined in claim 1 wherein said switching meanscomprises a semiconductor switch having an input, and triggering meanshaving an output for supplying a firing signal to the input of thesemiconductor switch, said triggering means having an input fed by theoutput of a differential amplifier.

8. A control system as defined in claim 7 wherein said triggering meanscomprises a blocking oscillator.

9. A control system as defined in claim 7 wherein said triggering meanscomprises a circuit incorporating a reed relay.

10. A control system as defined in claim 7 wherein said power supply hasan AC. voltage waveform.

11. A control system as defined in claim 10 wherein there is provided acommon gating signal source feeding a gating signal line in said bus,and wherein there is provided for each controller an AND gate having afirst input connected to the output of the amplifier and an out-putconnected to the input of said triggering means, said gate having asecond input connected to said gating signal line, said gating signalcomprising a short enable pulse at the start of each half-cycle of saidmains power supply.

12. A control system as defined in claim 10 wherein the period of saidperiodically varying electrical signal is equal to a half-period of theAC. voltage waveform of said power supply, and the waveforms aresynchronized so that phase angle control of the firing of eachsemi-conductor switch can be obtained.

13. A control system as defined in claim 1 wherein said switching meanscomprises a relay having a relay coil and relay contacts, one end ofsaid relay coil being connected to an amplifier the other end of therelay coil being connected to a common power supply line, there beingprovided means for permitting the relay coil to be energized for onlyone sense of said deviation.

14. A control system as defined in claim 13 wherein said meanspermitting the energizing of said relay coil comprises a diode connectedin series with said relay coil.

15. A control system for individually controlling the application of apower supply whereby each of a number of processes can be controlled inaccordance with the deviation of the value of a sensed variableparameter of each process from a common setpoint for the processes,which system comprises a multi-line bus, a controller for each processin the form of a unit connected to said bus, a common power supplysource for the controllers feeding common power supply lines in saidbus, a common set-point signal source, a common perturbation signalsource, summing means for combining said set-point signal and saidperturbation signal and for feeding a combined signal onto a setpointline in said bus, each controller comprising a high gain differentialamplifier having a first input, a second input, and an output, a sensingdevice providing a process variable parameter signal, means forconnecting said variable parameter signal to said first input saidsecond input being connected to said set-point line in said bus, andswitching means for controlling the application of said power supply toone of said processes,

9 said switching means having a control input fed by the output of saiddifferential amplifier.

16. A control system as defined in claim wherein there are providedmeans for sensing a disturbance of said power supply for said processesand for feeding a first common correction signal to said summing means.

17. A control system as defined in claim 15 where there are providedmeans for averaging said deviations, for deriving a second commoncorrection signal from said averaged deviations and for feeding saidsecond common correction signal to said summing means.

18. A control system as defined in claim 16 wherein

1. A control system for individually controlling the application of apower supply whereby each of a number of processes can be controlled inaccordance with the deviation of the value of a sensed variableparameter of each process from a common set-point for the processes,which system comprises a multi-line bus, a controller for each processin the form of a unit connected to said bus, a common power supplysource for the controllers feeding common power supply lines in saidbus, a common set-point signal source feeding a set-point line in saidbus, a common perturbation signal source supplying a periodicallyvarying electrical signal to a perturbation line in said bus, eachcontroller comprising a high gain differential amplifier having a firstinput, a second input and an output, a sensing device providing aprocess variable parameter signal, means for connecting said variableparameter signal to said first input via a first resistance, means forconnecting said perturbation line to said first input via a secondresistance, said second input being connected to said set-point line insaid bus, and switching means for controlling the application of saidpower supply for one of said processes, said switching means having acontrol input fed by the output of said differential amplifier.
 2. Acontrol system as defined in claim 1 wherein there are provided meansfor sensing a disturbance of said power supply for said processes andfor feeding a first common correction signal to a correction signal linein said bus, and means for connecting said correction signal line to thefirst input of each controller via a third resistance.
 3. A controlsystem as defined in claim 1 wherein there are provided means foraveraging said deviations, for deriving a second common correctionsignal from said averaged deviations, and for feeding said second commoncorrection signal to another correction signal line in said bus, andmeans for connecting said correction signal line to the first input ofeach controller via another third resistance.
 4. A control system asdefined in claim 1 wherein there is provided for at least one controllera variable resistance sensor having one of its two ends connected via afourth resistance to one of said common power supply lines, its otherend connected via a fifth resistance to another of said common powersupply lines, and a sixth resistance connected between one end of the oreach sensor resistance and said first input of the or each ampliFier, sothat the output of the or each amplifier is of the required sense forcontrol of said parameter.
 5. A control system as defined in claim 4wherein said set-point signal source comprises a potentiometer havingone of its ends connected by a seventh resistor to said one of saidcommon power supply lines and its other end connected by an eighthresistance to said another of said common power supply lines, the commonset-point signal being fed to the set-point line in said bus from aslider of the potentiometer, and wherein said other end of the or eachsensor is connected to said other end of said potentiometer via a linein said bus.
 6. A control system as defined in claim 1 wherein there isprovided for at least one controller a thermocouple having at least onehot and cold junction, the thermocouple having an output voltage fed tothe first input of the amplifier, and means for combining a coldjunction compensation voltage with said set-point signal.
 7. A controlsystem as defined in claim 1 wherein said switching means comprises asemiconductor switch having an input, and triggering means having anoutput for supplying a firing signal to the input of the semiconductorswitch, said triggering means having an input fed by the output of adifferential amplifier.
 8. A control system as defined in claim 7wherein said triggering means comprises a blocking oscillator.
 9. Acontrol system as defined in claim 7 wherein said triggering meanscomprises a circuit incorporating a reed relay.
 10. A control system asdefined in claim 7 wherein said power supply has an A.C. voltagewaveform.
 11. A control system as defined in claim 10 wherein there isprovided a common gating signal source feeding a gating signal line insaid bus, and wherein there is provided for each controller an AND gatehaving a first input connected to the output of the amplifier and anout-put connected to the input of said triggering means, said gatehaving a second input connected to said gating signal line, said gatingsignal comprising a short enable pulse at the start of each half-cycleof said mains power supply.
 12. A control system as defined in claim 10wherein the period of said periodically varying electrical signal isequal to a half-period of the A.C. voltage waveform of said powersupply, and the waveforms are synchronized so that phase angle controlof the firing of each semi-conductor switch can be obtained.
 13. Acontrol system as defined in claim 1 wherein said switching meanscomprises a relay having a relay coil and relay contacts, one end ofsaid relay coil being connected to an amplifier the other end of therelay coil being connected to a common power supply line, there beingprovided means for permitting the relay coil to be energized for onlyone sense of said deviation.
 14. A control system as defined in claim 13wherein said means permitting the energizing of said relay coilcomprises a diode connected in series with said relay coil.
 15. Acontrol system for individually controlling the application of a powersupply whereby each of a number of processes can be controlled inaccordance with the deviation of the value of a sensed variableparameter of each process from a common set-point for the processes,which system comprises a multi-line bus, a controller for each processin the form of a unit connected to said bus, a common power supplysource for the controllers feeding common power supply lines in saidbus, a common set-point signal source, a common perturbation signalsource, summing means for combining said set-point signal and saidperturbation signal and for feeding a combined signal onto a set-pointline in said bus, each controller comprising a high gain differentialamplifier having a first input, a second input, and an output, a sensingdevice providing a process variable parameter signal, means forconnecting said variable parameter signal to said first input saidsecond input being connected to said set-point line in said bus, andswitching means for controlling the application of said power supply toone of said processes, said switching means having a control input fedby the output of said differential amplifier.
 16. A control system asdefined in claim 15 wherein there are provided means for sensing adisturbance of said power supply for said processes and for feeding afirst common correction signal to said summing means.
 17. A controlsystem as defined in claim 15 where there are provided means foraveraging said deviations, for deriving a second common correctionsignal from said averaged deviations and for feeding said second commoncorrection signal to said summing means.
 18. A control system as definedin claim 16 wherein there are provided means for averaging saiddeviations, for deriving a second common correction signal from saidaveraged deviations and for feeding said second common correction signalto said summing means.
 19. A control system as defined in claim 15wherein there is additionally provided for at least one controlledprocess a further controller having its switching means connected to analarm.
 20. A control system as defined in claim 1 wherein saidperiodically varying electrical signal is a triangular voltage waveform.